An Outside Vapor Deposition (OVD) device is broadly used for making an optical fiber preform since it may give a bigger-diameter preform with high deposition efficiency.
An example of the conventional OVD device is schematically shown in FIG. 1. Referring to FIG. 1, the conventional OVD device includes a cylindrical burner 12 mounted upon a plate 10, and a mandrel 18 mounted above the burner 12 to rotate in a predetermined direction. While the OVD process is conducted, material particles constituting an optical fiber preform 16 are deposited on the mandrel 18. In the OVD process, the cylindrical burner 12 supplied with combustion gas and reaction gas emits flame 14 toward the mandrel 18 in order to cause a high temperature state thereto, and it is also reciprocated in the horizontal direction. This process causes generation of fine particles of the material constituting the optical fiber preform, and the generated particles are deposited on the surface of the mandrel 18 in a predetermined thickness.
More specifically, combustion gases such as H2 and O2 and reaction gases such as SiCl4 and GeCl4 are supplied to the cylindrical burner 12 at a predetermined flow rate. Then, combustion reaction of the combustion gases causes a high temperature state, and material particles such as SiO2 and GeO2 are generated. The generated particles are deposited on the surface of the rotating mandrel 18 in a predetermined thickness.
The material particles such as SiO2 and GeO2 are generated when the reaction gases are hydrolyzed with a burning product H2O or directly oxidized at or above 1100° C. with a carrier gas O2 formed by the burner 12 according to the chemical reaction formula expressed below. The fine particles of SiO2 and GeO2 collide into each other and condense into particles with a diameter of about 0.2 μm, and are deposited on the surface of the rotating mandrel 18.
Chemical Reaction Formula 1
SiCl4+2H2O→SiO2+4HCl (hydrolysis)
SiCl4+2O2→SiO2+2Cl2 (oxidization)
The deposition mechanism of the fine material particles constituting the optical fiber preform in the optical fiber preform manufacturing process using the OVD device is thermophoresis. Thermophoresis means that, when fine particles exist in a gas having a temperature gradient, the particles move from a high temperature area to a low temperature area due to the momentum exchange between particles and gas molecules. The rate of the thermophoresis is calculated according to the following Mathematical Equation 1.Vt=−(Kv/T)/ΔT  Mathematical Equation 1
Here, K is a thermophoresis constant.
As shown in the above Mathematical Equation 1, it will be known that the temperature gradient is a main factor to the particle deposition in the optical fiber preform making process using the OVD device. In other words, the combustion of hydrogen and oxygen emitted from the burner 12 makes the reaction gas be oxidized and the reaction gas by hydrolyzed by the flame near the burner 12 to form fine material particles constituting the optical fiber preform 16, and these particles move together with hot gases emitted from the burner 12 and pass around the mandrel 18. These particles are then deposited to the mandrel 18 having a relatively low temperature due to the effect of temperature gradient. Thus, the particle deposition efficiency is increased as the particle has higher temperature and the mandrel 18 has lower temperature.
In the OVD process, whenever moving on the plate 10, the cylindrical burner 12 changes compositions of SiCl4 and GeCl4 so that the optical fiber preform 16 may obtain a desired refractive index, in the general OVD process. In addition, the mandrel 18 is separated and removed from the preform 16 when the preform 16 has a desired deposition thickness. This preform 16 is then dried and sintered in a furnace which is maintained at a temperature of 1400˜1600° C., so as to make an optical fiber preform.
When executing the OVD process, one or multiple burners 12 are arranged in series, and then the burner(s) 12 or the mandrel 18 is moved laterally. It is because the burner 12 used in the conventional OVD process has a cylindrical shape as shown in FIG. 2, and thus heats just a local area of the optical fiber preform 16, as shown in FIG. 2.
On the other hand, FIG. 4 shows planar distribution of the flame 14 generated by the conventional cylindrical burner 12 equipped in the conventional OVD device. Referring to FIG. 4, it may be understood that the flame 14 is more focused in the center of the burner 12. In the planar distribution of FIG. 4, darker area shows that the flame 14 is more concentrated. Accordingly, the material particles deposited on the mandrel 18 by the conventional cylindrical burner 12 have a density gradient in a radial direction. This fact is also proven by FIG. 3 which shows soot particle density in X- and Y-direction.
If the material particles of the optical fiber preform 16 are deposited on the rotating mandrel 18 with laterally moving the burner 12, a spiral deposition pattern 19 is created on the surface of the mandrel 18, as shown in FIG. 5. This spiral deposition pattern 19 makes a deposition layer of a certain thickness whenever the burner 12 passes, and such deposition layers are stacked to form the optical fiber preform 16. However, due to the above-mentioned spiral deposition pattern 19, portions having a high soot density are overlapped at a certain position, and there are also generated non-overlapped portions on the mandrel 18. Thus, the overlapped portion 19a becomes relatively thicker, and the preform may hardly obtain a uniform thickness all over the length. In addition, rapid transfer of the burner 12 or the preform 16 may cause turbulence to a laminar flow of the flame, so there is a limit in improving the deposition efficiency. Moreover, ends of the optical fiber preform 16 cannot be used because of irregularity of the particle stream, thus causing losses.
The difference of the soot density may cause irregularity of the deposited thickness, and such irregular deposition may cause overlaps. Such overlaps become a factor limiting a deposition speed, a deposition amount and a deposition density as the optical fiber preform 16 increases, and eventually form ripples on an outer circumference of the finished optical fiber preform 16 after the sintering process. The ripples formed on the surface of the optical fiber preform 16 cause inferiority in the frequency blocking characteristic and the distribution characteristic which are sensitive to the core diameter, so the ripples should be removed.
To solve such a problem, there is proposed U.S. Pat. No. 4,486,212 entitled “Devitrification Resistance Flame Hydrolysis Process”. This document discloses a technique to guide silica particles to be uniformly deposited on the mandrel by decreasing an initial deposition speed to a very low level. Although the patented method may restrain the amplification of the irregular deposition to some extent, but it cannot solve the above problem completely. As disclosed in Korean Patent Filing No.92-19778, this technique cannot overcome the drawback that a loss at both ends of the optical fiber preform reaches 20% due to reciprocation of the burner or preform, and the control of deposition is difficult. In addition, the above conventional techniques cannot solve problems derived from the spiral deposition pattern generated after the initial deposition, and inevitably leads to lower productivity caused by the initial deposition control.
In addition, U.S. Pat. No. 4,683,994 entitled “Process and Apparatus for Forming Optical Fiber Preform” suggests a technique using a large-scale soot generator which cover all area of the optical fiber preform. However, when using the method proposed in the above document, gases are mixed inside the soot generator, so problems such as soot growth and clogging of nozzles due to the grown soot arise. In addition, since the overall optical fiber preform gets increased temperature, the particle deposition efficiency using the thermophoresis is even decreased.